From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754139AbbKBVLY (ORCPT <rfc822;w@1wt.eu>);
	Mon, 2 Nov 2015 16:11:24 -0500
Received: from mail-io0-f180.google.com ([209.85.223.180]:33637 "EHLO
	mail-io0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751633AbbKBVLW (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 2 Nov 2015 16:11:22 -0500
MIME-Version: 1.0
In-Reply-To: <8722.1446496609@warthog.procyon.org.uk>
References: <20151102132901.157178466@infradead.org>
	<20151102134941.005198372@infradead.org>
	<CA+55aFwMREaiVSZhTt3HB+Qrgj1pA5p9P6q8WD-dSa7=rHYPmQ@mail.gmail.com>
	<8722.1446496609@warthog.procyon.org.uk>
Date: Mon, 2 Nov 2015 13:11:22 -0800
X-Google-Sender-Auth: iB7BIpXS0mnIJcy_X1TzklLhw2Y
Message-ID: <CA+55aFzheg4-ahv-rRWqn0PqwjwyTToQZaYF4voQKDLACYeZ+A@mail.gmail.com>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Howells <dhowells@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>, boqun.feng@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, Michal Hocko <mhocko@kernel.org>,
        Will Deacon <will.deacon@arm.com>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Nov 2, 2015 at 12:36 PM, David Howells <dhowells@redhat.com> wrote:
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> > +       smp_read_barrier_depends(); /* ctrl */  \
>> > +       smp_rmb(); /* ctrl + rmb := acquire */  \
>
> Doesn't smp_rmb() imply an smp_read_barrier_depends() anyway?

Yes, it does. But that "smp_read_barrier_depends()" is actually
mis-used as a "barrier against subsequent dependent writes, thanks to
the control flow". It's not protecting against subsequent reads -
which is what the smp_rmb() is about.

Which is completely bogus, but that's what the comment implies.

Of course, on alpha (which is where smp_read_barrier_depends() makes a
difference), both that and smp_rmb() are just full memory barriers,
because alpha is some crazy sh*t. So yes, a "smp_rmb()" is sufficient
everywhere, but that is actually not where the confusion comes from in
the first place.

                   Linus